1. Field of the Invention
The present invention relates to a liquid crystal display construction including semiconductor devices which are made of a crystalline silicon film and act as control devices and, more particularly, to the construction of pixel regions of an active matrix liquid crystal display.
2. Description of the Related Art
In recent years, techniques for fabricating thin-film transistors (TFTS) on cheap glass substrates have evolved rapidly because there is an increasing demand for active matrix liquid crystal displays.
In an active matrix liquid crystal display, millions of pixels are arranged in rows and columns. At least one TFT is disposed at each of these pixels. Electric charge going into and out of each pixel electrode is controlled by the switching function of the TFT.
A liquid crystal material is sandwiched between each pixel electrode and a counter electrode to form one kind of capacitor. Accordingly, the electrooptical characteristics of the liquid crystal material can be varied by controlling the movement of the electric charge into and out of the capacitor by means of the TFT. Thus, the light transmitted through the liquid crystal panel can be controlled. As a result, an image can be displayed.
In the capacitor of this construction, the retained voltage gradually drops for leakage and other causes. This varies the electrooptical characteristics of the liquid crystal material, thus impairing the contrast of the displayed image.
Accordingly, it is common practice to place a separate capacitor, called retaining capacitor, in series with the capacitor comprising the liquid crystal material. This replenishes electric charge lost from the capacitor comprising the liquid crystal material due to the leakage or the like.
The structure of pixel regions of the prior art active matrix liquid crystal display is shown in FIG. 1(A), where gate lines 101 and capacitor lines 102 running parallel to the gate lines intersect data lines 103 like a matrix. Pixel electrodes 104 are disposed inside regions (hereinafter referred to as the pixel regions) surrounded by these lines. The capacitor lines 102 and pixel electrodes 104 are at different levels and overlap each other via first and second interlayer dielectric films, thus forming retaining capacitors.
A semiconductor layer 105 forms the active layer of a TFT. Indicated by 106 and 107 are contacts to the data lines and pixel electrodes, respectively.
In FIG. 1(A), the pixel regions surrounded by the gate lines 101 and data lines 103 which intersect each other at right angles act to display an image. It is required that the area of these pixel regions is made as large as possible.
In the structure shown in FIG. 1(A), it is necessary to form the capacitor lines 102 within the regions. This narrows the pixel regions accordingly. That is, the conventional structure has the disadvantage that the aperture ratio is low.
As shown also in FIG. 1(A), the pixel electrodes 104 are so formed as to overlap neither the gate lines 101 nor the data lines 103; otherwise parasitic capacitance would be created, thus adversely affecting the liquid crystal display, e.g., the operating speed of the liquid crystal display is lowered.
When a voltage is applied, the edges of the pixel electrodes 104 disturb the electric field, thus deteriorating the displayed image such as blurring of the image. Therefore, the edges are placed outside the field of view. That is, some contrivance is necessary to mask the edges.
Furthermore, it is necessary that the semiconductor layer 105 forming the active layer of the TFT be shielded against extraneous light, for the following reason. If light impinges on the semiconductor layer, its conductivity is changed by photoexcitation.
To provide such light shielding, a black matrix (BM) is normally formed either on the TFT substrate or on the counter substrate. A region which is within the field of view when the black matrix is disposed is shown in FIG. 1(B).
Referring to FIG. 1(B), all of gate lines 101, capacitor lines 102, data lines 103, and semiconductor layer 105 are covered with the black matrix to prevent them from lying within the field of view. Consequently, a region indicated by 108 is the effective image display region.
As described above, the capacitor lines 102 narrow the pixel regions excessively and lead to a deterioration of the aperture ratio.